1. Technical Field
The present disclosure relates to a sparkplug.
2. Description of the Related Art
In recent years, there has been an increased demand for the anti pre-ignition performance and the anti-fouling performance of a sparkplug due to a higher compression and a higher output of the engine.
The anti pre-ignition performance refers to a performance for suppressing excessive heating of the front end of the sparkplug to suppress the occurrence of the pre-ignition. The pre-ignition refers to a phenomenon in which an excessively heated front end of an insulator of the sparkplug serves as a heat source and thereby combustion starts spontaneously inside a combustion chamber of the engine before the ignition of the sparkplug.
The anti-fouling performance refers to a performance for suppressing the occurrence of a spark at a portion where carbon has been attached. Once a large amount of the carbon is attached near the front end of the insulator of the sparkplug, a current flows in the carbon. As a result, this may cause a leakage (a short circuit) phenomenon in which, instead of running between electrodes of the sparkplug, a spark runs at the portion where the carbon has been attached. The carbon that has attached to the front end of the insulator has the characteristics to burn out at around 520 degrees centigrade or higher. Thus, there has been proposed a sparkplug having a self-cleaning function that causes the carbon to burn out by its own heat by rapidly increasing the temperature up to around 520 degrees centigrade.
As described above, the anti pre-ignition performance is improved as the rise in the temperature of the insulator of the sparkplug is suppressed, while the anti-fouling performance is improved as the temperature of the insulator of the sparkplug rises. Therefore, it has been a problem that it is difficult to achieve both anti pre-ignition performance and anti-fouling performance of the sparkplug.
Conventionally, a technique disclosed in, for example, Japanese Patent Application Laid-open No. 2005-183177 has been known as the technique for achieving both anti pre-ignition performance and anti-fouling performance of the sparkplug. Also see Japanese Patent Application Laid-open No. 2002-260817.